The present invention relates to a showerhead electrode assembly for improved thermal uniformity in a semiconductor processing reactor. Specifically, the invention relates to an electrode plate removably clamped to a backing plate or heat sink plate using a compliant, thermally conductive electrostatic clamp. The invention also relates to a method of processing a semiconductor substrate such as a wafer with the showerhead electrode assembly.
Vacuum processing chambers are generally used for chemical vapor depositing (CVD) and etching of materials on substrates by supplying process gas to the vacuum chamber and application of an RF field to the gas. For example, parallel plate, transformer coupled plasma (TCP(trademark), also called ICP), and electron-cyclotron resonance (ECR) reactors are disclosed in commonly-owned U.S. Pat. Nos. 4,340,462; 4,948,458; and 5,200,232. During processing, the substrates are held in place within the vacuum chamber by substrate holders. Conventional substrate holders include mechanical clamps and electrostatic clamps (ESC), which clamp a wafer to a pedestal during processing. The pedestal may form both an electrode and a heat sink. Examples of mechanical clamps and ESC substrate holders are provided in commonly-owned U.S. Pat. Nos. 5,262,029, 5,740,016, and 5,671,116 and in U.S. Pat. No. 6,292,346. Substrate holders in the form of an electrode can supply radiofrequency (RF) power into the chamber, as disclosed in U.S. Pat. No. 4,579,618.
Electrostatic clamps secure a substrate to a pedestal by creating an electrostatic attractive force between the substrate and the clamp. A voltage is applied to one or more electrodes in the ESC so as to induce opposite polarity charges in the substrate and electrode(s), respectively. The opposite charges pull the substrate against the pedestal, thereby retaining the substrate.
In a monopolar design, the ESC comprises an electrode, a workpiece and a dielectric between them. In the normal mode of operation for a monopolar ESC the electrode is connected to the negative pole of a DC power supply. The workpiece is connected through the plasma to ground. Under this arrangement the workpiece is not securely clamped prior to exposing the workpiece to the plasma or other return path.
In a bipolar configuration the ESC can clamp the workpiece absent a plasma; a second pole of the ESC provides the return path. This configuration uses both the positive and negative potential of the DC power supply to electrostatically clamp the workpiece.
A tripolar ESC contains three poles. The inner two poles are similar to the bipolar configuration, and are used to clamp the workpiece. The outer pole can be used as either a plasma shield or a workpiece bias pickup point. U.S. Pat. No. 5,572,398 discloses a tri-polar electrostatic clamp using separate positive and negative electrodes housed on a non-polarized base.
Multi-pole ESC""s use either AC or DC that is phased to each pole of the electrode. The phasing of the voltage applied to the electrode permits rapid clamping and release. The supply and control circuitry required for a multi-pole type of ESC is more complicated than for the monopole, bipolar or tripolar configurations. The multi-pole configuration is used to minimize charge build up on the workpiece and also helps reduce de-clamping issues.
The materials and processes used in processing a semiconductor substrate such as a wafer are extremely temperature sensitive. Excessive temperature fluctuations in the substrate may compromise system performance and the resulting properties of the semiconductor device. Thus, in order to adequately control the temperature of the substrate, good thermal contact between the substrate and the substrate holder is desired.
To facilitate heat transfer between the substrate and substrate holder, a very large electrostatic or physical force is commonly used in an attempt to cause the greatest amount of wafer surface to physically contact a support surface of the substrate holder. Due to surface roughness of both the substrate and the holder, however, when the two surfaces are pressed against each other only point contacts are established; i.e., small interstitial spaces (voids) constitute the majority of the interface. Under low pressure processing conditions, this interface is evacuated and the voids comprise a vacuum, which is a very good thermal insulator. Thus, heat transfer between the two surfaces is limited mainly to the point contacts.
To improve temperature uniformity across the substrate during processing, an inert, high thermal conductivity gas such as helium is pumped into the interstitial spaces formed between the substrate and the support surface. This heat transfer gas acts as a more efficient thermal transfer medium between the substrate and the substrate holder than the vacuum it replaces.
Some ESC devices are designed to minimize escape of heat transfer gas into the surrounding low pressure atmosphere (i.e., the reaction chamber). The support surface in such a device can have a circumferential raised rim having a diameter that is approximately equal to the diameter of the wafer and a flex circuit covering the support surface of the underlying pedestal. The flex circuit is usually a conductive material encased in a flexible dielectric material. The conductive material is patterned to form the electrostatic electrode. The dielectric material insulates the conductive material from other conductive components and also acts as a gasket. Once the wafer is clamped, a gas tight seal is created between the wafer and the rim. As such, heat transfer gas leakage from beneath the wafer at the rim is minimized. An electrostatic substrate holder comprising a lip seal for clamping substrates is disclosed in commonly-owned U.S. Pat. No. 5,805,408. Electrostatic substrate holders comprising flex circuits are disclosed in U.S. Pat. Nos. 6,278,600 and 6,033,478.
As described above, adequate wafer temperature control can be obtained through backside He gas pressure and electrostatic clamping techniques. In addition to substrate temperature control, however, temperature control of all the reactor surfaces that come into contact with active process chemistry is desirable for process repeatability and uniformity. Various upper electrode heating and cooling designs have been developed and are being used in semiconductor processing apparatus, for example parallel plate dielectric etchers. A reaction chamber component having improved temperature uniformity is disclosed in commonly-owned U.S. Pat. No. 6,123,775.
As shown schematically in FIG. 1, in some prior art designs the upper electrode plate 150 is held against a temperature controlled heat sink plate 110 via a backing ring 120. The electrode 150 is a planar disk having uniform thickness from center to edge thereof. The temperature of the heat sink plate is maintained by circulating a heated or cooled liquid through channels 114 within the plate. The heat sink plate is furnished with a process gas feed 134 for supplying gas to the process chamber. The gas then is dispersed through a plenum 140 and passes through gas dispersion holes (not shown) in the electrode to evenly disperse the process gas into the reaction chamber. According to this design, most of the thermal contact between the electrode 150 and the heat sink plate 110 is established at the periphery of the electrode where force from a peripheral mechanical clamp 160 is directly applied. The advantage of this technique is that the electrode plate, which is a consumable part, is separable from the heat sink plate. The disadvantage, however, is that the limited peripheral contact between the electrode and heat sink plate results in large center-to-edge temperature non-uniformity in the electrode.
As shown in FIG. 2, the center-to-edge temperature non-uniformity problem of limited thermal contact has been addressed in designs that use a backing plate 220 that can be bonded or brazed to the main electrode 250. The electrode plate/backing plate assembly are held in contact with heat sink plate 210 using clamping bolts 264. As in the previous example, the temperature of the heat sink plate 210 is maintained by circulating a heated or cooled liquid through channels 214 within the plate. The heat sink plate is also furnished with a process gas feed 234 for supplying gas to the process chamber. The gas is distributed through one or more horizontally spaced apart plenums 240 and passes through gas dispersion holes 254 in the backing plate and in the electrode (not shown) to evenly disperse the process gas into the reaction chamber. Because the whole assembly (electrode plate 250 and backing plate 220) become a consumable part, however, such an approach is expensive to implement. An electrode clamping assembly wherein the electrode is mechanically and removably attached to the backing plate is disclosed in commonly-owned U.S. Pat. No. 5,569,356.
Accordingly, there is a need for an economical electrode design that provides improved thermal uniformity.
The present invention provides an electrode arrangement that improves the heat transfer between a backing plate and a showerhead electrode plate by positioning a compliant material between the two interfaces. The compliant material conforms to the surface irregularities of the two abutting surfaces and provides better thermal contact. The showerhead electrode includes an electrostatic chuck that further improves the thermal contact by applying a compressive force between the electrostatic plate and the electrode plate.
According to one aspect of the invention, the apparatus for retaining an electrode plate in a semiconductor processing chamber comprises a backing plate having an electrode plate receiving surface; an electrostatic holding apparatus disposed upon the electrode plate receiving surface, the electrostatic holding apparatus having an electrode plate support surface; an electrode plate having a lower surface facing a semiconductor substrate to be processed and an upper contact surface; and a mechanical clamping member engaging the outer periphery of the electrode and pressing the upper contact surface against the electrode plate support surface.
The electrostatic holding apparatus of the invention can comprise a first dielectric layer, a conductive layer disposed below the first dielectric layer, and a second dielectric layer disposed below the conductive layer. The first dielectric layer, the conductive layer, and the second dielectric layer can comprise at least one process gas port. Preferably, the first and second dielectric layers comprise a compliant material such as, for example, silicone or polyimide. The conductive layer of the electrostatic holding apparatus can comprise aluminum, copper, titanium, tungsten, molybdenum, nickel, silver, gold, iridium, platinum, ruthenium, ruthenium oxide, graphite, titanium nitride, titanium aluminum nitride, titanium carbide, or combinations thereof.
The electrostatic holding apparatus can have a thickness of 0.005 to 0.015 inches and can have a bipolar or multipolar design. Preferably, the electrostatic holding apparatus can further comprise a heating element to provide resistive heating of the electrode plate. According to a preferred embodiment, the electrode plate can comprise a showerhead electrode and the backing plate and electrostatic holding apparatus can include at least one process gas port for supplying a process gas to the plasma reaction chamber. The electrode plate can comprise single crystal silicon, graphite, or silicon carbide. Preferably, the ratio of the area of the electrostatic holding apparatus to the area of the upper contact surface is greater than 80%.
The invention also relates to a method of assembling a showerhead electrode assembly of a plasma reaction chamber. The method includes arranging a backing plate having an electrode plate receiving surface, and an electrostatic holding apparatus with an upper surface adhesively attached to the electrode plate receiving surface, the electrostatic holding apparatus having an electrode plate support surface opposite the upper surface. The method further includes arranging a showerhead electrode plate having a lower surface facing a semiconductor substrate to be processed and an upper contact surface such that the upper contact surface faces the electrode plate support surface; engaging a mechanical clamping member with the outer periphery of the showerhead electrode plate and pressing the upper contact surface against the electrode plate support surface; and applying a voltage to the electrostatic holding apparatus to electrostatically attract the showerhead electrode plate to the electrode plate support surface.
According to a preferred method, the electrostatic holding apparatus can comprise a first compliant dielectric layer, a conductive layer disposed below the first compliant dielectric layer, and a second compliant dielectric layer disposed below the conductive layer, the second compliant dielectric layer having an electrode plate support surface such that the electrode plate support surface conforms to the upper contact surface when the showerhead electrode plate is electrostatically attracted to the electrode plate support surface. The process also includes arranging the backing plate, electrostatic holding apparatus, and showerhead electrode plate so as to include at least one process gas port for supplying a process gas to the plasma reaction chamber. According to a preferred process, the electrostatic holding apparatus can further comprise a heating element to provide resistive heating of the showerhead electrode plate.
The invention also provides a method of processing a semiconductor substrate such as a single wafer in a plasma reaction chamber. The method includes transferring a semiconductor substrate into the plasma reaction chamber; supplying a process gas to the plasma reaction chamber; applying a voltage to an electrostatic holding apparatus to electrostatically attract an electrode plate to an electrode plate support surface during processing of the substrate; and supplying electrical power to the electrode causing the process gas to form a plasma for processing an upper surface of the substrate. The method can be used to etch a layer of material on the substrate, such as a silicon dioxide layer. The method can also be used to deposit a layer of material on the substrate. The method can also include supplying electrical power to the electrostatic holding apparatus to heat the electrode plate during processing of the substrate.